This invention relates to a process for obtaining fructose and other useful products from starting materials containing the fructose moiety or monomeric unit, e.g. sucrose-containing or other saccharides, especially of the fructofuranoside type. An aspect of this invention relates to a method for separating fructose and glucose when these sugars are obtained by hydrolysis of such starting materials. A further aspect of this invention relates to various chemical precipitation steps useful for separating the monosaccharide hydrolysis products from each other.
Many research studies have reported that fructose is sweeter than sucrose (ordinary sugar). According to Shallenbeiger et al, SUGAR CHEMISTRY, page 116 (1975), the AVI Publishing Company, Inc., crystalline fructose is 1.8 times sweeter than sucrose. Accordingly, fructose is fast becoming one of the most popular candidates for sweetening foods and beverages--its greater sweetening power making possible a significant reduction in the caloric intake of the food or beverage consumer. In recent years, a number of synthetic sweeteners have come under close scrutiny as a result of experiments indicating carcinogenic activity in experimental animals; hence, the purely "natural" route to lower calorie intake offered by fructose sweetening has acquired even greater significance. Indeed, some researchers claim a variety of physiological benefits can be obtained by including fructose in the diet.
Fructose (also called "levulose" in some of the earlier scientific literature) is widely distributed in nature. In addition, several readily available materials such as enzymatically derived and isomerized corn syrup and honey can contain significant amounts of this sugar. Nevertheless, in many aspects of the industrial practice of manufacturing pharmaceuticals, foods, beverages, dietary supplements, and the like, these various relatively low fructose-content sugars (typically 42 to 55% fructose) are not preferred. One somewhat more preferred form is a "high fructose syrup", i.e. a relatively concentrated aqueous solution of substantially pure fructose or fructose mixed with minor amounts of other carbohydrates, which can, if desired, by crystallized to directly obtain substantially pure crystalline fructose.
Much of the crystalline fructose produced for today's market is obtained from raw materials containing a mixture of sugars. The fructose is separated from the mixture and purified using techniques such as ion exchange and chromatography. Another technique, less widely used, involves chemical precipitation of fructose with alkaline earth metal complexes. This technique takes advantage of the fact that fructosate complexes are less soluble in water than, for example, the corresponding glucosates. Still another approach to the manufacture of fructose involves isomerization of monosaccharide isomers such as glucose, e.g. with sodium aluminate or the enzyme of glucose isomerase.
At first glance, the chemical precipitation technique (whereby alkaline earth metal cations form complexes with the sugars in the sugar mixture) would appear to be very promising. The disaccharide sucrose can be considered to be an equimolar combination of glucose and fructose, since acid or enzymatic hydrolysis of the sucrose molecule can provide an equimolar mixture of these two monosaccharides (in any of their isomeric forms). A very promising aspect of this process stems from the very high level of availability of sucrose, not only in pure form, but also in impure sugar-bearing liquors, juices and various by-product forms such as molasses. Apparently, significant progress in the utilization of the chemical precipitation technique for separating fructose from glucose was hindered by the stability of the alkaline earth metal fructose complex under cold conditions. Various acids will break up this complex and release the fructose, the most common of these being carbonic acid. Both beet sugar and cane sugar manufacturing facilities, typically, have an ample source of relatively pure carbon dioxide available; so carbonic acid has traditionally been considered to liberate fructose from the generally water insoluble complexes (e.g. calcium fructosate). See, for example, U.S. Pat. No. 2,007,971 (Jackson), issued July 16, 1935, particularly page 3, line 31 et seq. As noted by Jackson, the carbon dioxide as carbonic acid causes a precipitation of calcium carbonate and releases fructose to the aqueous medium. The calcium carbonate precipitate can then be removed by filtration.
The results of this carbonic acid precipitation technique have apparently not met modern industry standards for the production of relatively pure fructose. For example, even at low temperature, a considerable amount of color bodies tend to form prior to or during or even subsequent to the liberation of the fructose from the fructosate complex, meaning destruction of the fructose molecule. In many large scale uses of fructose (or sugars generally), a clear solution or a pure white powder or crystal is desirable or even essential for consumer acceptance or for satisfying industry-imposed quality control standards. Consequently, these color bodies must be removed--or their formation avoided. Furthermore, whenever one is dealing with fructose and glucose, it is necessary to take into account their pH and heat sensitivity; the heat produced by exothermic reaction may be sufficient to accelerate degradation of these rather sensitive sugar products.